Momentum of electron

find the speed of an electron.
given that the momentum of the electron is 4.8 x 10-19kg.m/s

p=mv/ sqr root of 1-(v/c)2

use the above equation to find v.
The solution in the textbook re-writes the above equation as
p=(mc)(v/c)/sqr root 1-(v/c)2. How on earth do they come to this, and how is this equation solved. been sitting with this for a long time and very confused.